Heretofore in this field, automatic controllers have been used to regulate the temperature of a cooking medium, such as oil or shortening, in a cooking appliance, such as a deep fat fryer. Typical controllers are based on microprocessors which execute software programs stored in associated memory, such as read-only memory (ROM), for example. The microprocessor and ROM may be contained on the same integrated circuit, but this is not necessary. The microprocessor typically has a plurality of input/output (I/O) ports operable to receive data indicative of the state of the fryer and/or the cooking medium and further operable to output control signals to the fryer. In a typical system, the temperature of the cooking medium is sensed by a temperature sensing probe and this temperature is input to the control system. The control system then compares this temperature reading with the desired cooking medium temperature and outputs a control signal which turns on or turns off a burner or electrical heating element. The desired cooking medium temperature may be placed into the ROM during manufacture of the controller or during use in the field, if provision has been made in the design of the controller for programming by the user. This desired temperature is normally expressed as a "set point", or target temperature.
Advanced fryers require relatively sophisticated automatic controllers to keep the cooking medium at the set point primarily because the temperature of the cooking medium is significantly lowered when uncooked food is placed into the cooking medium. This effect is particularly problematic when frozen food is placed into the cooking medium. The large temperature differential between the frozen food and the hot cooking medium enables a significant amount of thermal energy to be quickly transferred from the cooking medium to the food, lowering the temperature of the cooking medium below the set point. The amount of temperature reduction is unpredictable and depends on such factors as the quantity of food, the food temperature, the effective surface area of the food, the temperature of the basket which holds the food, and many other factors. The control system is designed to return the cooking medium temperature to the set point as rapidly as possible while minimizing the overshoot of the set point.
A significant problem in prior art cooking device controllers has been the large amount of overshoot encountered while attempting to arrive at the set point. The problem arises when heat is applied to the fryer to compensate for a lowering of the cooking medium temperature caused by the uncooked food being placed into the cooking medium. The indicated temperature at the temperature sensing probe normally rises a significant amount of time after the heat is applied to the cooking medium. This lag time is due to the thermal transit time through the cooking medium from the heat source to the temperature sensing probe. During heating, this inertia in the temperature sensing system causes the indicated temperature of the cooking medium to be less than the actual temperature of the cooking medium closer to the heating element. Therefore, if the heat is removed from the cooking medium once the set point temperature is reached at the temperature sensing probe, the temperature lag time results in the temperature of the cooking medium at the probe location continuing to rise and overshooting the set point. The converse is also true. When heat is removed from the cooking medium, its temperature near the heating element falls faster than the indicated temperature of the temperature sensing probe. The indicated temperature of the cooking medium is therefore higher than the actual temperature. The inertia in the temperature sensing system will cause the indicated temperature of the cooking medium to continue to fall even after heat is reapplied to the system. The indicated temperature will not begin to rise again until all of the thermal masses between the heating element and the temperature sensing probe are thermally charged.
A number of different prior art algorithms have been written using proportion control in an effort to limit temperature overshoot and still achieve a rapid return to the set point under different load conditions. These programs have had less than desirable success because of the slow response or necessarily poor location of the temperature sensing probe.